Field of the Invention
The present invention relates to a method of producing a casting mold, and the casting mold. More particularly, the invention relates to a method of producing a casting mold using a molding material mixture in a dry state or a wet (moisture) state, and a casting mold produced by the method.
Description of Related Art
As one type of a method of producing a casting mold used for casting a molten metal, the method of forming self-curing molds by using inorganic binders like a water glass as a binder has been disclosed. However, the casting mold formed by using the inorganic binder consisting of the water glass suffers from deterioration of its strength due to moisture absorption, resulting in a defect of a poor moisture-resistant strength of the casting mold. Thus, the casting mold has a problem of difficulty of its use in a highly humid environment. Even though there are some kinds of a water glass which have a good moisture-resistant strength, these kinds of water glass tend to be poor in formability. Accordingly, conditions of such available water glass may be limited, so that measures to improve both of the formability and the moisture-resistant strength are desired.
In view of the above, JP-A-2008-511447 suggests one of the methods of producing a mold material by using a fine particulate material formed by mixing a heat-curable binding composition as an inorganic binder. The method disclosed in this document uses at least one refractory mold raw material and a molding mixture for producing a metal processing mold including at least a binder based on a water glass. To the molding mixture, a particulate metal oxide, which is selected from the group consisting of silicon dioxide, aluminum oxide, titanium oxide and zinc oxide, is added at a predetermined ratio. Addition of the particulate metal oxide is considered to provide advantages of an improvement of initial strength of a mold, namely the strength of the mold immediately after removal from a hot device, and an improvement of the moisture resistance.
However, such molding mixture hardly exhibits an effect of improvement of the moisture resistance where the metal oxide is used in a small amount, so that it is necessary to add the metal oxide in the amount larger than that of a solid content of the water glass. For this reason, such molding mixture does not solve the problem of deterioration of formability, even though it may improve the moisture-resistant strength. In particular, a silicon dioxide causes generation of free silicic acid in a step of mixing of foundry sand or in a step of reclaiming of waste foundry sand, with the free silicic acid resulting in deterioration of the working environment, or even a harmful effect to the respiratory organs of the human body. Furthermore, although the molding mixture permits a short-term improvement of the moisture resistance, it cannot sufficiently solve the problem that the strength of the casting mold decreases with a passage of time due to moisture absorption, in the case where the casting mold is held in a highly humid atmosphere for a long time, for example. Specifically, in the case where the formation of the casting mold is performed at a temperature of 200° C. or lower, even conventional additives for improving the moisture resistance do not permit a sufficient strength of the mold subjected to moisture absorption.